So far we've learned that E runs are to develop your aerobic system to improve your endurance and that R runs are to train your body to run fast. E runs are supposed to feel very comfortable and conversational, and R runs are supposed to be fast and make you breathe hard, but they're short and you recover fully before trying to run fast again. But, if your goal is to RACE faster, you can't run easy and you can't take recovery breaks! Threshold Pace (T) is a key training pace to train your body to be able to hold a faster pace for a longer duration.
When running, the cells in your muscles are going through very complex chemical reactions to create the energy needed to run. These processes result in the production of metabolic waste. The faster you run, the more and more quickly waste is produced. When this waste accumulates, your legs feel heavy, burning, and uncomfortable, and it's very difficult, if not impossible, to continue running. T pace running is meant to be fast enough to create more waste than would be created with E running, but not so fast that your body can not clear the waste products as they are produced. When you find this sweet spot and spend some time practicing it, your body becomes more efficient at clearing the waste. Over time, as your body becomes more efficient at clearing waste, you can hold faster paces for longer periods of time, and your race times improve!
Daniels states that this pace should feel "comfortably hard." When running the correct pace you should feel like you could maintain the pace for 30-40 minutes if you really had to, but you are working hard enough that you look forward to the run's completion; you may be able to converse in short, one-word responses, but you really want to tell whoever is talking to you to shut up. Contrast this with E pace at which you should feel like you could easily run and hold a conversation for many miles.
The VDOT table has been updated with what your personal T pace should be. It's vitally important not to go too fast when doing T paced work. Remember, the point of it is to clear waste products at roughly the same rate as they are being produced in order to prevent their accumulation. If you go too fast, the waste will accumulate and you won't be able to complete the recommended time at T pace. The point is missed entirely. Don't run any faster than necessary to achieve to benefits of the workout. That just leads to fatigue and burnout. So, when you're supposed to be running T pace you should honestly assess yourself in the first few minutes of the run and ask, "could I hold this pace for 30-40 minutes if my life depended on it?" If the answer is no, slow down. Resist the urge to "race" the workouts. Trust me, I've done that and it leads to nothing but misery. So let me repeat that key point one more time: Don't run any faster than necessary to achieve the benefits of the workout.
Improving your running takes a lot of time and trial and error. The VDOT table and suggested paces don't guarantee that you'll dramatically improve, but they provide a springboard to organize your training. Increasing your overall mileage is also an important step to improving your running. Again, improving takes a lot of time. It's a long-term commitment. If your goal is to get faster, be patient, stick to the process, and after committing to a period of training, do a race and then re-assess your training and your goals. If nothing else, you're getting some exercise and having some fun, right? Stick with it ;-)